Asynchronous transfer mode (ATM) networks carry fixed sized cells within the network irrespective of the applications being carried over ATM. At the network edge or at the end equipment, an ATM Adaptation Layer (AAL) maps the services offered by the ATM network to the services required by the application. There are a number of industry standards and proposed standards covering various AALs. In particular, “B-ISDN ATM Adaptation Layer Type 2 Specification,” draft Recommendation 1.363.2, November 1996, of ITU-T (herein referred to as AAL2) provides for efficient ATM transport of small, delay-sensitive packets in such applications as packet voice systems. AAL2 is partitioned into two sublayers, the Common Part Sublayer (CPS) and the Service Specific Convergence Sublayer (SSCS).
In an AAL2/SSCS packet voice system, the peak required raw bandwidth of voice, coded in accordance with ITU-T standard embedded ADPCM G.727 (hereafter referred to as G.727), is 32 thousands of bits per second (kb/s). However, other types of voice-band type traffic are also carried in this system besides voice itself. For example, G3 facsimile (fax) traffic may be conveyed requiring a typical bandwidth of 9.6 kb/s. Also, data traffic may be carried with required bandwidths of as much as 64 kb/s in the case of 56 kb/s modem technology.
As a result, an AAL2/SSCS packet voice system multiplexes a variety of traffic types onto an outgoing ATM virtual circuit (VC) pipe, which has a fixed bandwidth allocation in accordance with an ATM service category, e.g., ATM Constant Bit Rate (CBR), ATM Real-Time Variable Bit Rate (rt-VBR). (This bandwidth is typically fixed, or static, and negotiated with a distant ATM endpoint at setup of the VC.) Once the VC is set up, new calls may be admitted to the VC in accordance with a call admission algorithm. In this call admission algorithm, all traffic is treated in a homogeneous fashion in one extreme. A new call is admitted simply by comparing the current number of calls in the respective VC to a predetermined call threshold value. If the current number of calls is less than this call threshold value, then the new call is admitted. Otherwise, the new call is blocked.
Unfortunately, as new calls are admitted to the pipe, traffic loads may necessitate the use of congestion relief algorithms for the voice traffic such as bit dropping or dropping entire AAL2 voice packets. (It is presumed that only voice traffic is throttled to relieve congestion and that non-voice traffic is not targeted for packet dropping in order to provide for congestion relief.) For example, as congestion begins to occur, voice packets are typically queued for transmission in a buffer, or queue. If the number of these queued voice packets exceeds a predetermined threshold, bit dropping for voice traffic begins to occur in accordance with, e.g., G.727. If the congestion continues to worsen, then entire AAL2 voice packets are dropped. (Also, it should be noted that if the above-mentioned thresholds are too small, bit dropping occurs too soon, and if the above-mentioned thresholds are too large, bit dropping occurs too late. In this latter case, there is almost little, or no, benefit from bit dropping (in the context of G.727) because of the already incurred large packet delay by the time bit dropping begins to start.)